Recently, because of a demand for a higher power of an engine and measures for an exhaust gas to minimize atmospheric pollution, use environment of the piston ring is becoming severer and wear of an outer peripheral surface of the piston ring has been a problem to be solved. To cope with such the problem, there has been provided a piston ring which has an outer peripheral surface thereof coated with a hard film which exhibits excellent anti-scuffing and wear resistance such as a film made of chromium nitride, titanium nitride or the like by ion plating in place of a conventional hard chromium plating film or nitriding treatment.
However, in a diesel engine using a piston ring with such a nitride on an outer peripheral surface thereof as a hard film, when the combustion pressure is high or when an exhaust gas countermeasure is taken by a large quantity of EGR (Exhaust Gas Recirculation), there arises the problem that the wear on the outer peripheral slide surface is increased. Accordingly, it is less than optimal to ensure the lifetime and the sealing performance of the piston ring for a long time. Particularly, with respect to an outer peripheral slide surface of a portion of butt ends of a top ring which define a gap, since a high temperature combustion gas passes a closed gap between the butt ends when compressed within a cylinder and hence, the lubrication condition is more serious compared to other portions. Further, since force pressing the outer peripheral slide surface of piston ring to an inner wall surface of the cylinder by combustion gas pressure is greater at butt ends than that at other portion of a piston ring, the wear of the outer peripheral surface at the butt ends is greater than that of the other surface.
Due to such wear, when the hard film formed on the outer peripheral slide surface of the butt ends or the portion thereof by ion plating is dissipated, a steel material which constitutes a substrate is exposed to a slide surface so that the wear rapidly progresses whereby the closed gap size between the butt ends is increased resulting in lowering of the sealing function.
Lowering of the sealing function attributed to the increase of closed gap size increases a quantity of blowby gas which is a combustion gas blowing to an oil pan side so that oil is degraded. Further, a lubricating oil leaks to a combustion chamber side and this brings about not only the increase of an oil consumption but also the occurrence of environmental problem such as the increase of sulfuric component in an exhaust gas or the like. Further, when the hard film is dissipated due to wear and the substrate material of steel is exposed, the danger of engine scuffing is also increased.
In view of the above, finding of a countermeasure to cope with the wear in the vicinity of the butt ends has been an important task to be solved. As the countermeasure to cope with the wear of the butt ends, in Japanese Unexamined Patent Publication No. 2000-120866, there has been proposed an idea in which a radial wall thickness in the vicinity of the butt ends is set smaller than that of other portion of a piston ring by approximately 20 to 30% and hence, a face pressure attributed to an own expanding force is decreased at the butt ends whereby the sealing ability is maintained and, at the same time, the wear of the butt ends is prevented.
However, with respect to a top ring, at a top dead point of a piston, explosive combustion pressure is applied to a back surface of the piston ring and hence, the butt ends are pressed to an inner peripheral wall of the cylinder with a pressure which far exceeds a pressure derived from the own expanding force. Accordingly, it is the fact that the proposal disclosed in the previously mentioned publication is also less than optimal as the countermeasure.
Further, in a high-load diesel engine which increases the combustion pressure or in an engine which adopts an exhaust brake, the piston ring is deformed in a dish shape at the time of explosive combustion or at the time of operating an exhaust brake and this is liable to induce wear of ring grooves of the piston. When the ring groove of the piston is worn, the dish-like deformation of the piston ring is enlarged. Due to this repeated deformation stress, the fatigue breakage of the piston ring is liable to be induced at a position opposite to the butt ends at which the stress is concentrated.
The hard film is poor in toughness and cracks are liable to easily initiate due to the stress concentration. Particularly, when a thickness of the hard film is increased, this tendency becomes more remarkable. Further, when the hard film is made of nitride formed by ion plating (for example, chromium nitride, titanium nitride or nitrides in which oxygen, carbon, boron or the like is added to those nitrides), there has been a problem that the film is peeled off due to fatigue of repeated friction force which is attributed to the sliding movement between the piston ring and an inner peripheral wall of the cylinder.
To prevent this film peeling caused by the frictional force, it is effective to intentionally introduce a compressive residual stress into the film by changing a bias voltage or the like in the ion plating process. In the ion plating, in general, the film has a tendency to grow and to be stacked while generating a compressive stress in a film forming process in reference to the substrate. However, when the compressive residual stress is made to remain in the hard film, the tensile residual stress is generated in a portion of the substrate material right below the film. When the piston ring is deformed in a dish shape, provided that an external stress and the residual stress are combined, a maximum tensile stress is generated right below the film in many cases. When a defect or the like exists in the vicinity of the portion, it functions as a starting point of fatigue failure and hence, there arises a new problem that the breakage of the piston ring is liable to occur easily. This tendency becomes more apparent corresponding to the increase of the thickness of the hard film.
Further, when the hard film is uniformly thick as mentioned previously, there arises a problem on a cost besides the above-mentioned problems. Particularly, when the hard film is formed by ion plating, ion plating equipment is expensive, the film forming speed is slow compared to that of other method such as electric plating or thermal spraying and hence, the productivity is low. Further, a target such as chromium or titanium is also expensive, the greater the film thickness, the piston ring becomes more costly.
Further, the shape of the piston ring is changed when microstructure of material is changed. The deformation is liable to occur in the direction that the curvature of the free shape of the butt ends of the piston ring is increased, that is, in the direction that the diameter of the piston ring is decreased.
Accordingly, film forming is performed at a temperature of approximately 500° C. which is equal to or below a tempering temperature of the substrate material. However, as mentioned previously, when the compressive stress remains in the film, the film is deformed even at a temperature which the microstructure change of the material does not occur and this deformation is remarkably large when the heat-resistant element in the substrate material is small in quantity. Accordingly, when a compressive residual stress is generated in the film, this tendency becomes particularly apparent along with the increase of the film thickness. Particularly, since the curvature of the outer peripheral portion in the vicinity of the butt ends is substantially equal to the nominal diameter of the piston ring, when the curvature of this portion increases (the curvature being continuously changed to decrease the diameter), a non-contact region is formed between the outer peripheral surface of the piston ring and the inner peripheral wall of the cylinder.
In the manufacturing process of the piston ring, there exists a lapping step in which, to finish the piston ring into predetermined nominal outer diameter size and shape, and further to smooth the surface roughness of the outer periphery of the piston ring, the piston ring is set inside a lapping sleeve having a predetermined inner diameter after performing film forming by ion plating and the outer periphery surface made of the film is polished. In the piston ring coated with the ion plating film, due to the above-mentioned reason, the non-contact region is liable to be easily formed on the outer peripheral portion in the vicinity of the butt ends in the lapping step. To obtain the complete contact between the outer peripheral portion of the piston ring and the inner peripheral wall of the lapping sleeve, it takes a considerable time for lapping and, in a worst case, even when the other film is dissipated, the contact in the vicinity of the butt ends cannot be obtained.
Further, since an inner peripheral surface of the sleeve for lapping is worn out in that case, the lifetime of the sleeve for lapping is extremely shortened resulting in the manufacturing of costly piston ring.